As device geometries continue to shrink, one of the challenges facing the semiconductor industry is the deposition of thin, continuous films for use as copper (Cu) diffusion barriers and as seeds for electroplating. A metallic liner like ruthenium (Ru), tantalum (Ta) or cobalt (Co) is needed over a barrier layer, for example, tantalum nitride (TaN), to improve adhesion and electromigration. Another area of need is the copper seed. While atomic layer deposition (ALD) of metal (carbon) nitride layers is well advanced, there are several challenges to growing elemental metallic layers by ALD or chemical vapor deposition (CVD). Some of the problems with growing thin, metal films by CVD or pulsed layer deposition include the incorporation of impurities from the organic ligands or reactant gas, poor nucleation and growth under non-oxidizing conditions, poor step coverage, and lack of continuity.
There are additional problems associated with metal films. For example, the deposition of Ru is difficult in a reducing environment; most ALD chemistries require oxygen or another oxidizing agent. Reducing chemistries have been used, but they typically either require “exotic” reducing agents or have large nucleation delays, that is, the delay in deposition between the time when a substrate is exposed to a metal precursor and the time when the metal actually begins to form on the substrate surface.
Other approaches require the intermediate deposition of or conversion to an oxide or nitride followed by the subsequent reduction to the elemental state. This type of method results in a metal film containing large amounts of contaminants, namely, oxygen or nitrogen, which increases the resistivity of the resulting film. Moreover, the ability to reduce a previously-deposited film is limited to a certain thickness because the reduction process has a finite, practical, penetration depth.